10.04.2017

Crawler crane incident report

The contactor of the site where the telescopic boom of a crawler crane came down in January has released the full details of the investigation.

The operator had lifted an 800kg compressor from the centre of the site area and had slewed right to place it elsewhere, as he began to lower the boom something broke and it began to descend rapidly outside of the operator’s control. He fortunately had the clarity of mind to slew back to the left and possibly retract the boom a little in order to prevent the boom coming down on to the boundary fence and a public footpath on the other side. His quick reaction meant that the boom came down just inside the site perimeter, with no one hurt or injured and minimal to zero material damage.
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The crane just before lifting the compressor

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The scene after the boom came down - the operator managed to slew and possbily retract the boom a little before it hit the bounadary wall

As previously reported the boom descended when the pressure transducer on the piston (lift) side of the boom lift cylinder was sheared off. It is possible that a chunk of wood left in the area was the cause (although the report does not conclude that), along with some localised corrosion to one of the primary fittings. The crane has two transducers one on the piston / elevate side of the cylinder and another on the rod or power/down side, both combine to provide an accurate load indication for the Load Moment Indicator.
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The holding valve set up, the transducer in the foreground is the rod side, the piston side transducer is on oposite side of the block

The transducers were, according to the initial report – an Hitachi Europe retrofit part - and comprised two male/male fittings, the transducer itself and a pressure snubber which removes shock loadings to the transducer, such as might be experienced as the crane tracks with a load. The net effect is a fairly long contraption that is attached to a block welded to the lift cylinder.
As the transducer needs access to the pressure side of the cylinder to work, oil will escape should it be knocked off as in this case.
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The transducer assembly was knocked off when male/male fitting into the block sheared

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The broken transducer

The full report will shortly be posted in the Vertikal.Net online Library

Vertikal Comment

Two things – maybe three - jump out from this report, One that storing chunks of wood within the crane superstructure is not a good idea. Keeping the entire crane tidy through good housekeeping is clearly the way to go. Two – that this valve had too many parts, in a perfect world the transducer would have a built in ‘Snubber’ and it would have a connector that allowed it to be directed attached to the block, rather than through an adapter. Finally the possible third is that according to the report the block of wood, taken from a pallet, was used to prop open a machinery cover. If the machinery cover had a built in prop or air cylinder – make do blocks would not be needed.

In summary plaudits should go to the contractor - St James and the rental company Eagle Crane Hire for making this information widely available as well as to the crane operator for his professionalism and quick reactions.